If you've ever been put under anesthesia, you might recall a disoriented, almost delirious sensation as you wake back up. That feeling is more than simple post-surgery confusion: it's actually the brain reverting to a more primitive evolutionary state.

That's the finding of researchers from UC Irvine and Finland's University of Turku, who used positron emission tomography, or PET, to image twenty research volunteers' brains as they reawakened from anesthesia. They found that consciousness - in this case, the patients' ability to perform a motor response when commanded to - first emerged as a "core network" of the brain reactivated. This network included the brain stem, thalamus, hypothalamus, and other ancient parts of the brain. As University of Turku Professor Harry Scheinin explains, this wasn't exactly what they thought they would see:

"We expected to see the outer bits of brain, the cerebral cortex (often thought to be the seat of higher human consciousness), would turn back on when consciousness was restored following anesthesia. Surprisingly, that is not what the images showed us. In fact, the central core structures of the more primitive brain structures including the thalamus and parts of the limbic system appeared to become functional first, suggesting that a foundational primitive conscious state must be restored before higher order conscious activity can occur."

The researchers used the drug dexmedetomidine, a sedative typically used in intensive care, and propofol, which is one of the most common drugs used to induce and maintain anesthesia. Dexmedetomidine was particularly important, according to the researchers, because its effects are similar to those of normal sleep, which allowed the researchers to more readily determine whether brain activity was shaped by the effects of the drugs themselves or the actual shifts in consciousness.

Advertisement

Both drugs triggered the same responses from the brain's ancient core network, which suggests primitive consciousness really is localized in this part of the brain. The exact implications of this are hard to determine right now, but they could well be huge. After all, if these parts of the brain - which evolved long, long before Homo sapiens emerged - can produce even a very basic state of consciousness, then we need to understand just how neural mechanisms manage to do that...and how many other species might experience a similar level of consciousness.